1. Technical Field
An aspect of the present invention relates to an imaging device, an imaging method, an electronic apparatus, and the like suitable for combining plural kinds of picked-up image data having different exposures and generating image data having an expanded dynamic range.
2. Related Art
In the related art, there is a technique for imaging the same subject with plural kinds of exposures (exposure times), combining pixel signals obtained by the imaging respectively corresponding to the plural kinds of exposures, and generating image data having an expanded dynamic range (HDR image data). In such a technique, noise (a false color, a pseudo contour, etc.) often occurs in a boundary area between an image portion corresponding to each of the exposures and image portions corresponding to the other exposures in a combined image.
For example, it is assumed that the same subject is imaged in exposure times T1, T2, and T3 (T1<T2<T3) corresponding to exposures L1, L2, and L3 (L1<L2<L3) and pixel signals S1, S2, and S3 of light response shown in FIGS. 8A to 8C are obtained. In this case, as shown in FIGS. 8D to 8F, first, with reference to the pixel signal S3 having the longest exposure time, the pixel signals S1 and S2 are multiplied by coefficients “T3/T1” and “T3/T2” to normalize the pixel signals S1 and S2. The normalized pixel signals “S1×T3/T1” and “S2×T3/T2” and the pixel signal S3 are linearly combined. Consequently, as shown in FIG. 8G, a picked-up image signal having an expanded dynamic range (having high S/N) is obtained.
However, during imaging, in some case, images are picked up by different light responses in a set exposure time because of an external factor such as offset fluctuation. In such a case, it is likely that the coefficient values “T3/T1” and “T3/T2” are inappropriate and the linearity of a picked-up image signal after combination is distorted. When the linearity is distorted, noise as a cause of a false color, a pseudo contour, or the like occurs in connecting portions of a polygonal line shown in FIG. 11.
In order to solve such problems, in JP-A-7-131708 (Patent Document 1), an accurate light response gradient of a picked-up image is calculated. In JP-A-2001-352552 (Patent Document 2), gradients are corrected to coincide with each other taking into account an OB area (an offset level) of a camera. In JP-A-08-214211 (Patent Document 3), an accurate ratio of exposures is calculated by analyzing a histogram of an image. As a method of not analyzing a characteristic of an image, in JP-A-2004-48445 (Patent Document 4), a low-pass filter is applied near a boundary area of a combined image to relax unnaturalness.
Even when accurate exposures (exposure ratios) of images are known and the images are multiplied by an appropriate coefficient to be combined, if a ratio itself of T1:T2:T3 during imaging is inappropriate, in some case, a noise component due to an S/N limit of a sensor occurs in an image after the combination.
For example, as shown in FIGS. 8D to 8G, if exposure times are set in a relation in which a noise region (a wavy line portion in the figure) of the pixel signal S1 is supplemented by the pixel signal S2 and a noise region of the pixel signal S2 is supplemented by the pixel signal S3, the problem does not occur. However, as shown in FIGS. 12D to 12G, when the exposure time T2 is slightly short, the noise region of the pixel signal S2 cannot be sufficiently supplemented by the pixel signal S3 and noise occurs near a boundary between the pixel signals S2 and S3.
However, in related arts including Patent Documents 1 to 4, there is no method for theoretically and dynamically determining ratios of exposures with respect to optimization of multi-stage exposure including three or more stages.